Seismic Facies and Sedimentology of Terrigenous Pleistocene Deposits in Northwest and Central Gulf of Mexico1
Charles J. Stuart, Charles A. Caughey, 1977. "Seismic Facies and Sedimentology of Terrigenous Pleistocene Deposits in Northwest and Central Gulf of Mexico", Seismic Stratigraphy — Applications to Hydrocarbon Exploration, Charles E. Payton
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Clastic sedimentary facies in the northern and central Gulf of Mexico were delineated using borehole and seismic data. The present continental shelf is underlain by deltaic deposits which appear as intervals of discontinuous, strong reflectors on seismic profiles. Fluvial, prodelta, and transgressive marine deposits are characterized by weak reflectivity. Near the present shelf edge, rapid progradation of mud and sand formed wedges of inclined layers (inclined-reflector seismic facies).
The continental slope of the northwest Gulf of Mexico is strongly affected by salt tectonics. Thick sedimentary sequences preserved between massifs, stocks, and spines of salt are characterized by interbedded, continuous, strong reflectors and irregular chaotic units. Turbidity-current and hemipelagic settling mechanisms initially formed these deposits, followed by downslope mass movements and bed disruption. The structure and thickness of sedimentary layers ; on the slope vary according to sedimentation rates, upward diapiric movement, and erosion. At least one deep erosional valley, the Mississippi Trough, formed on the slope during a low stand of sea level. The trough probably formed by slumping, bottom-current, and turbidity-current mechanisms, and is thought to have channeled sediment from the shelf to the Mississippi fan.
Slope deposits grade downslope into the continental- rise deposits. The Mississippi fan, the major rise feature in the Gulf of Mexico, consists of widespread chaotic layers interbedded with strong-reflector intervals. Some of the interbedded strong reflectors are folded or are terminated by slump faults, suggesting that rapidly deposited, low-strength mud was disrupted repeatedly by gravity processes.
Continental-rise deposits grade into abyssal-plain sequences in the deepest part of the gulf. Undeformed deposits represented by intervals of continuous, strong reflectors underlying the abyssal plain onlap or interfinger with the adjacent slope and rise deposits. The undeformed layering was not affected by salt diapirism or faults; these deposits were formed by turbidity-current and pelagic processes.
This study of Pleistocene deposits suggests that seismic profiles can provide data of Stratigraphic and sedimentologic importance. Similar interpretive techniques may also provide geologically useful data in other basins.
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Papers from a research symposium at the 1975 American Association of Petroleum Geologists and supplemented by later reports became “Seismic Stratigraphy Applications to Hydrocarbon Exploration”, one of AAPG’s best-selling book publications. Dramatic improvements in seismic imaging were demonstrated, a result of developments in seismic data quality and the processing capability of electronic technology. Twenty-eight articles are grouped into three sections. The first describes principles that both permit and also limit interpretations. The second section presents sixteen articles that describe the qualitative approach to stratigraphic interpretations of reflection records, and the final section presents techniques and examples of modeling. Of particular interest are a series of eleven papers in the second section under the subject heading of “Seismic stratigraphy and global changes of sea level”. Prepared by P. R. Vail, R. M. Mitchum and others from Exxon, they describe the regional unconformities and stratigraphic changes resulting from sea level fluctuations, and the manner in which these changes can be interpreted from seismic surveys. For many individuals within the oil industry who purchased this book, it was their first introduction to the modern concept of sequence stratigraphy that would have a major impact on the methodology of petroleum exploration.